1. Field of the Invention
The present invention is directed to an internal mold release composition suitable for use in the production of optionally cellular, polyurethane elastomers or optionally cellular, rigid structural polyurethanes by the reaction injection molding (RIM) process.
2. Description of the Prior Art
Polyurethanes having an impervious outer skin (integral skin) and an optionally cellular core are well known and suitable for many uses. Such polyurethanes are generally produced by introducing a reaction mixture based on polyisocyanates, isocyanate-reactive compounds and other additives into closed molds such that the reaction mixture fills the mold and reproduces the inner surface thereof. In order to prevent the molded article from adhering to the surface when the mold is opened, the mold is generally coated with a release agent. Examples of release agents currently in use include waxes, soaps, and oils. These external release agents form a thin film between the surface of the mold and the molded article. This thin film assists in preventing the molded article from adhering to the mold and thus enables the article to be readily removed from the mold.
This method has a number of disadvantages where mass production is desired. For instance, the release agent has to be applied at regular intervals, and during that period the mold is out of service. Also, fine engravings on the mold surface such as imitation wood or leather grain, become covered with residues of release agents over a period of time. The removal of these firmly adhering residues from very complicated molds involves considerable time and expense. Further, the molded articles are also coated with a thin film of release agent which causes adhesion problems when subsequently painted.
One solution to this problem has been to include a mold release agent in the reaction mixture so that it is possible to reduce or eliminate the separate application of an external mold release agent. U.S. Pat. No. 4,076,695 discloses the incorporation of carboxy functional siloxanes into the polyurethane reactants to be used in a reaction injection molding process in order to aid the release of the molded article from the mold. One difficulty with this approach is that the carboxy functional siloxanes react with the tin catalysts conventionally used to promote polyurethane formation resulting in insoluble adducts which no longer provide mold release properties.
Various solutions have been suggested to overcome this problem. U.S. Pat. No. 4,379,100 discloses a three stream system wherein the polyol component is divided into two portions, one containing the tin catalyst and the other containing the carboxy functional siloxane mold release agent. A second method of overcoming the problems with the use of carboxy functional siloxanes is disclosed in U.S. Pat. No. 4,420,570 wherein the tin catalyst is blended with the polyisocyanate component, while the carboxy functional siloxane is blended with the polyol component in order to keep them separate.
In a third solution to the problem, U.S. Pat. No. 4,396,729 discloses replacing the conventional high molecular weight polyol with an aminated polyol. Since no hydroxyl groups are present, it is not necessary to use an organo metallic catalyst and, thus, these catalysts are not present to interact with the carboxy functional siloxanes. U.S. Pat. No. 4,472,341 discloses a fourth solution to this problem wherein the acid group of the carboxy functional siloxane is chemically derivatized to form a new compound which does not interact with the organo metallic catalyst.
Even though these solutions presented in the above-identified patents do solve the problem of the interaction between the carboxy functional siloxane and the tin catalyst, there are disadvantages to these solutions. For example, it is very difficult to accurately meter three streams and, thus, this system is not used commercially. The addition of tin catalysts to the polyisocyanate component reduces the storage stability of this component, while the use of amino polyethers excludes the use of hydroxyl group-containing reactants. Finally, the formation of derivatives of the carboxy functional siloxanes reduces the mold release effects of these compounds.
Accordingly, it is an object of the present invention to provide mold release compositions which are storage stable, provide multiple releases and do not require special reactants or apparatus for processing. It is also an object of the present invention to provide mold release compositions which do not degrade the properties normally associated with these optionally cellular polyurethane elastomers. Surprisingly, these objects may be achieved in accordance with the invention described hereinafter.